Sensitized notched transducers



y 1967 L. K. RUSSELL ETAL 3,320,568

SENSITIZED NOTCHED TRANSDUCERS Filed Aug. 10, 1964 2 Sheets-Sheet 1 I PB r F 1 F 2 37 F/G. 6 40 r 3/ I/Vl/E/VTORS LEW/.5 A. RUSSELL W/LHELM H.LEG/17' United States Patent 3,32%,568 SENSITEZED NGTCHHLD TRANSDUCERSLewis K. Rossetti, Livermore, Wiihelm H. Legat, Woodside, and Roy C.Hackiey Hill, San Jose, (Jalifi, assignors to Raytheon Company,Lexington, Mass, a corporation of Deiawarc IFiied Aug. 10, 1964, Ser.No. 388,412 4 Gaiters. (CL 538-2) The present invention relates tosemiconductor signal translating devices and, more particularly, tosensitized notched transducers and methods for making the same.

This invention sets forth a semiconductor signal translating device of anew and improved form and is predicated upon the discovery thatnonuniform, concentrated, anistropic stress on junctions can be detectedand interpreted in terms of the current, voltage or reactancecharacteristics of such junctions. These types of signal translatingdevices are described in copending applications, Ser. No. 261,065, filedFeb. 26, 1963 and Ser. No. 268,- 772, filed Mar. 28, 1963. Both of theseapplications have been assigned to the assignee of the instantapplication.

The term junction as used herein is defined asa region of transitionbetween semiconducting regions of different electrical properties-whichdefinition was established by IRE standards in the October, 1954 issueof the Proceedings of the IRE.

The term point defects refers to that portion of applicants inventionwhich comprises various imperfections in the crystalline lattice of thesemiconductor material. These point defects may be formed by suchdiverse methods as: scribed lines, rapid quenching from hightemperatures, diffusion of various materials (such as gold) with asubsequent quenching, irradiation by particle beams (such as electronbeams), sandblasting and even polishing with an abrasive.

In terms of process, the invention relates to the preferential etchingof stressed or deformed semiconductor material by an acidic etchant. Interms of structure, the invention comprises a body of crystallinesemiconductor material having a junction therein, which material has aplurality of point defects adjacent a first side of the junction and anotch which is adjacent the reverse side of the junction in oppositionto the point defects. The opposing notch provides means for producing aconcentrated, nonuniform, anisotropic stress within a small region ofthe junction. The point defects and notch coact to effect a much higherresponse than previous transducers, e.g., as high as 1.74 mi-croam psper dyne-centimeters. Thus, the inclusion of these point defectsprovides a sensitized device, as that term is used herein.

The theory of operation of this invention, however, is not completelyknown. It is believed that the point defects produce dislocation loopsdeep within the junction, carrying generation-recombination centers withthem. These dislocation loops in the crystalline lattice are preventedfrom collapsing (With increased temperature) by being pinned to thedamaged surface created by the point defects. Under conditions of nobending moment the reverse characteristic of the diode is alreadydegraded; that is, a generation current is superimposed over the normaldiode reverse leakage current. Thus, when a bending force is applied tothe device such that the notch is narrowed, the dislocation loops arecaused to collapse or be forced towards the point defects. This, inturn, pulls generadon-recombination centers out of the depletion layerand decreases the generation current below its zerostress value. When abending force is applied to in the opposite direction, the notch iswidened which causes the dislocation loops to penetrate deeper into andtoward the depletion layer carrying with them a greater number ofgeneration-recombination centers. This serves to increase the generationcurrent.

In an illustrative embodiment of this invention a planor, sensitizednotched transducer comprises a high resistivity substrate (approximatelyfifty ohms-centimeter) of single crystalline silicon of one conductivitytype about 20 x x 8 mils. An opposite conductivity type layer isproduced on the substrate by any well known method such as diffusion.Ohmic contacts are maintained in electrical communication with both thediffused layer and the substrate in a known manner, such asthermocompression bonding. Point defects are then formed in the diffusedlayer by one of the previously mentioned methods and an opposing backnotch is cut into the substrate by an acidic etching technique that willbe more fully described below. The resulting device operates atfrequencies between zero and forty kc. (it also has D.C. sensitivity).Therefore, this device may be used in any piece of equipment Where it isdesired that mechanical force or pressure be converted into anelectrical signal. Since the device has high sensitivity, only a smallnumber of amplification steps are necessary to obtain a usuable signal.

It is thereof an important object of the present invention to provide animproved semiconductor transducer having a higher sensitivity thanprevious transducers.

A further object of the invention is to provide means for producingdislocation loops in a region of a junction within a semiconductortransducer.

Yet another object of the present invention is to provide means forconcentrating nonuniform, anisotropic stress in a small region of ajunction Within a semiconductor transducer.

A still further object of the instant invention is to provide an acidicetching technique for forming notches in semi-conductor materials.

FIGS. 1-5 represent successive operations in the present improved methodof forming a notch in a body of semiconductor material;

FIG. 6 shows a longitudinal sectional view of a sensitized notchedtransducer according to this invention;

FIG. 7 is an enlarged view of a portion of FIG. 6;

high frequencies;

FIG. 9 depicts a wide-notched or channeled transducer having an opposingsensitized area;

FIG. 10 illustrates a sensitized notch transducer in which an ohmiccontact is alloyed into the notch; and

FIG. 11 illustrates a transistor embodiment of the device shown in FIG.10.

In a previous device utilizing the anisotropic strain employed to,alternatively, rectify material or vary the net resistance of thematerial. This type of device is more fully set forth in the previouslymentioned copending application, Ser. No. 261,065.

Later embodiments of these devices achieved the desired stressconcentration by notching the semiconductor element adjacent the P-Njunction. These notched devices, together With a method of etching thenotches therein by means of a base material such as sodium hydroxide(NaOH), are set forth in the aforementioned copending application, Ser.No. 268,772. However, as more fully set forth hereinafter, the presentinvention is notched by means of an improved acidic etching tech- Thenotch formed by this acidic technique is preferred in certainapplications due to the fact that it can be one-fourth the width of thenotch formed by NaOH and yet remain the same depth. For example, notcheshave been formed in semiconductor material by the present acidictechnique which are 2.2 mils in width at their widest point and 8 milsdeep. Notches formed by the NaOH method which are 8 mils deep, areusually over 9 mils wide at their widest point.

Referring now to FIG. 1, there is disclosed a semiconductor body ordiode 11 of suitable material, such as silicon, germanium, or galliumarsenide, comprising a junction 12 which separates a region of p-typeconductivity material 13 from a region of N-type conductivity material14. Preferably, body 11 is composed of a chip of 111 crystalographicoriented semiconductor material such as silicon. N-type dopants cancomprise boron or iodine. P-type dopants can comprise phosphorous oraluminum.

nique.

Junction 12 in body of semiconductor material 11 may initially be formedby the method set forth in US. Patent No. 3,025,589 which issued to J.A. Hoerni on Mar. 21, 1962, entitled, Method of ManufacturingSemiconductor Devices. For example, one satisfactory device may beformed by diffusing a P-type impurity into an N-type silicon wafer. Withan N-type silicon wafer, the impurity would be one of the known acceptorimpurities, preferably alloyed with silicon. Application of sufficientheat to raise the wafer to an appropriate temperature results in adiffusion of the impurity into the Wafer so as to produce a region orportion of P-type silicon within the wafer. Intermediate the two typesof silicons now forming the wafer, there is produced the previouslydefined junction. As silicon technology is available in the literature,it is here only noted that N-type silicon may be formed by inclusion ofan impurity chosen from Group V of the Periodic Table while P-typesilicon may be formed by inclusion of an impurity from Group III.

Although the instant inventive process will be illustrated withreference to a diode type semiconductor body, it is within the scope ofapplicants process that various semiconductor devices, includingtransistors, could also be utilized.

FIG. 2 shows the semiconductor body 11 after it has been masked with anetch resistant film 16, e.g., beeswax. This masking operation may beperformed by dipping semiconductor body 11 in a solution of beeswax anda petroleum solvent, such as toluene =or xylene, until the entireworkpiece is covered by the solution. It has been found that thissolution best promotes masking at a temperature of about 80 C. Themasking operation is then concluded by removing the semiconductor body11 from the solution and drying it in air at room temperature.Microscopic examination of the work is then conducted to disclose anypossible pin holes that may be present in film or coating 16. If holesare observed, the foregoing operation is repeated until a uniformhole-free coating 16 is achieved as shown in FIG. 2. It is to be notedthat although beeswax is preferred, it is within the scope of applicantsprocess that coating 16 be composed of any substance which is known toresist etching.

Thereafter, as shown in FIG. 3, a groove 17 is scribed through layer 16.This scribing operation removes a strip of the film 16 down to thesurface of P-type conductivity material 13 and at the same time theapplied pressure damages body 11 in such a manner that its latticestructure is both deformed and stressed. Damaging is preferablyaccomplished by scribing with a diamond stylus having a pointed tip.However, this method of masking all of the surface except that portionwhich is exposed through groove 17 with the etch resistant coating 16and damaging the semiconductor material 11 is by way of illustrationonly. A variety of techniques known to the art of manufacturingsemiconductor devices 4- may be employed for these purposes withoutdeparting from the spirit of the present invention.

As shown in FIG. 4, a notch 18 is next formed in P-type conductivitymaterial 13 adjacent groove 17. Notch 18 is achieved by placing coatedand scribed semiconductor body 11 in an etching tank and submerging samein a mechanically agitated, acidic etchant. This acidic etchant may becomposed of any of the acidic silicon etches known to the art, such aseither 35 or 4-1 ratios of hydrofluoric (HF) and nitric (HNO acids.

In some instances it has been found advantageous to adjust the etchingrate by diluting the etchant with deionized water or by adding acetic(HC H O acid. In this manner the etchant preferentially etchessemiconductor body 11 along the damage formed by the scribing processrather than laterally across its surface. The width and the depth ofnotch 18 is dependent upon such parameters as: the composition of theetching solution, the width of scribed groove 17 and the time length ofmechanical agitation of the etching solution. When the desiredconfiguration is achieved, NaOH is mixed (in large quantities) into theetchant to quench the etching action. Semiconductor body 11 is thenremoved from the etching tank and washed in deionized water. The notcheddevice may be then cleaned with a petroleum solvent (to remove etchresistant layer 16) and dried.

In certain applications, however, a kerf-like notch is desired inmaterial 13. As shown by the dotted lines in FIG. 5, the envelope ofsuccessively etched grooves 19 form a kerf 21. To form this kerf, thefollowing method and technique is utilized. First, the notch 18, asshown in FIG. 4, is formed by the previously described process. Then,rather than cleaning with petroleum solvent and drying, wax coatedsemiconductor body 11 is heated until layer 16 flows into notch 18. Aportion of the wax now adhered to the bottom of notch 18 is then scribedaway and the etching operation is repeated on the thus exposed portionof semiconductor body 11 for a shorter time period than that used toform notch 18. These operations are then repeated as often as requiredto achieve the desired geometry and dimensions. From the abovedescription, a new technique or method has been described for producingnotches or grooves in semiconductor materials.

With reference to FIG. 6, there is shown a cantileveredtype, sensitizedstrain transducer 31 comprising a body of semiconductor material 41)having a junction 41 therein. and held in a cantilevered position by asupport means 32. Support means 32 may comprise a first electrode 33 incontact with a surface 34 of semiconductor body 40. Electrode 33 ismaintained in a fixed relationship to a second electrode 35 by means ofan insulator 36. The electrodes 33 and 35, together with insulator 36,provide a support means whereby body 40 is held in a cantileveredposition and whereby opposing surfaces 34 and 37 of the body aremaintained in electrical communication with electrical communicationwith electrodes 33 and 35, respectively. Support means 32 is shown as anelectrode device by way of illustration only; it is entirely within thescope of the present invention that the support means be composed of anelectrically insulating material while electrical contact is provided bysome other means.

The major contribution of the instant invention resides in the fact thatthe semiconductor body 40 has both point defects 38 (introduced in amanner to be described hereinafter), adjacent junction 41, and anopposing notch or groove 39 etched into surface 37. As more fully setforth in the previously mentioned copending applications, it ispreferred that junction 41 lie within a relatively shallow depth (e.g.0.00002 inch) of surface 34. Notch or groove 39 may be in the form of akerf or a V-shaped groove or any other suitable reduction in at leastone dimension of body 40, such as perpendicular to the plane of surface34, so that anisotropic stress will be applied to a small area ofjunction 41. It is preferable that notch 39 be located relatively closeto the clamped end of body 40 with its apex near junction 41 so that aforce applied along the line shown by force arrow 42 or 43 will takeadvantage of the mechanical leverage to cause anisotropic stress toappear in junction 41 at the apex of groove 39. The net effect is avariation in the electrical characteristics of the junction inproportion to the magnitude and direction of the force applied.

As shown in the enlarged portion of FIG. 6 depicted in FIG. 7, straintransducer 31 further comprises a plurality of point defects 38 adjacentthe pinnacle of groove 39. These point defects can be engendered by avariety of means. For instance, in the aforementioned x 120 x 8 mils.illustrative embodiment of this invention, point defects 38 were formedin the crystalline lattice of the semiconductor body 40 by scribing aline into (and thus damaging) surface 34 in opposition to notch 39. Thisproduces dislocation loops within the junction and pins thesedislocation loops to the damaged surface created by the scribed line.

These point defects can also be formed in such semiconductor materialsas silicon by rapidly quenching the silicon from high temperatures. Asstated on pages 3663- 3665 of the December 1962 issue of the Russianjournal Fizika Tverdogo Tela, volume 4, No. 12, this quenching processcan be achieved by quenching the semiconductor material from 1200 C.This process comprises: heating the material to 1200 C., maintainingthis temperature for twenty to sixty minutes, and then injecting thematerial into an oil bath to cool the material at a rate not less than10 -10 C./second. An alternative quenching proc ess, one in which thesemiconductor material is heated in a nitrogen atmosphere for severalhours and then quenched rapidly, is described in an article entitled,Quenched-In Recombination Centers in Silicon, by G. Benski which waspublished in volume 103, No. 3, in the August 1, 1963, issue of PhysicalReview.

Applicants have also found that point defects 38 may be formed byirradiation with particle beams, e.g., electron beams. As disclosed inElectron-Bombardment Damage in Silicon by G. K. Wertheim which waspublished in volume ll. 0, No. 6, of Physical Review on June 15, 1958,this irradiation can be accomplished by electron bombardments in thevacuum of a Van de Graaff accelerator at a variety of temperatures andat bombarding energies of 1.0 and 0.7 mev. Applicants have also foundthat these point defects are enhanced by diffusion of various materials,such as gold or copper, followed by the aforementioned rapid quenching.An illustrative example of this technique is taught by ThermalGeneration of Recombination Centers in Silicon by B. Ross and J. R.Madigan published on December 15, 1957, in volume 108, No. 6, ofPhysical Review. Thus, applicants have found that a variety of means maybe used to create the crystalline impurities or interstices within asemiconductor material which comprise point defects 38.

FIG. 8 depicts an alternate embodiment of strain transducer 31. In thisembodiment, a semiconductor body 54 having a notch 55 and point defects56 adjacent a junction 57 is supported at one end by support means 58. Athin vibrating element 51 is adhered to the nonsupported end of thesemiconductor body 54 to provide a vibrating element which greatly aidshigh frequency response. Thus. the device will convert high frequencyvibrations (above 25 kc.) along force line 52 or 53 with close toperfect fidelity.

FIG. 9 illustrates yet another embodiment of a cantilevered sensitizedstrain transducer. In this embodiment, a semiconductor body 69, suitablysupported at one end by support means 61 comprises a channel 62 adjacentjunction 63 wherein said channel substantially defines a trapezoidalgroove having a planar base surface 64 generally parallel to junction 63across an appreciable area thereof. Thus the term channel, as usedherein, refers to a wide-notched groove having at least three sides, oneof which is substantially parallel to junction 63. All other elements ofsemiconductor body 60 are substantially the same as those of straintransducer 31. However, point defects 65 are now formed over anappreciably wider area and may be produced by such methods assandblasting surface 66 adjacent channel 62 or polishing surface 66 withan abrasive or scribing a plurality of relatively close parallel linesinto surface 66 adjacent channel 62.

Referring now to FIG. 10, there is disclosed a sensitized straintransducer according to the present invention in which a semiconductorbody 70 has a notch and point defects 76 as described, and in which anotched side ohmic contact 71 is alloyed directly into notch 75. Ohmiccontact 72 is then alloyed to the opposing side of semiconductor body70. This provides a better current path from terminal 73 to terminal 74by maximizing the current through the sensitized region of the device,thereby avoiding additional recombinations outside the space charge(depletion) region. Terminals 73 and 74 are preferably compressionbonded gold lead wires.

FIG. 11 shows an embodiment of a sensitized strain transducer transistor81 similar to the device of FIG. 10, but wherein the body 82 ofsemiconductor material has two junctions 83 and 84 therein. Straintransducer 81 employs the ohmic contact arrangement as disclosed in FIG.10. The notch 86 is located adjacent only the single junction 84. Forthis reason, when force is applied along either direction indicated byforce arrow 87 or 88, the concentrated, nonuniform, anisotropic stressproduced by the coaction of this force with notch 86 affects thecharacteristics of junction 84, only. This provides a higher responsefor junction 84 without affecting the properties of junction 83. For thePNP transistor shown, it is preferred that alloyed ohmic contacts 89 and90 are composed of aluminum while base ohmic contact 91 is composed of agold containing N-type impurity.

It should be noted that many changes in this structure as shown in thedrawing and described in the specification may be made within the scopeof the present invention. For example, while the above description ofthe transistor structure has been referenced to a PNP transistor, itwill be appreciated that it is equally applicable to a PNP typetransistor. Further, while cantilevered embodiments are shown, otherconfigurations such as thin diaphragms are also possible. Accordingly,it is to be understood that the form of the present invention is to betaken as a preferred example of the same and that various changes in theshape, size, material, constitution, and arrangement of parts may beresorted to without departing from the spirit of said invention or thescope of the subjoined claims.

What is claimed is:

1. A device comprising (a) a body of semiconductor material having afirst region of selected conductivity type and a second region ofopposite conductivity type, said first and second regions havingrespective first and second oppositely disposed planar surfaces, and aP-N junction therein between said regions and extending in a planeparallel with one of said surfaces,

(b) a groove extending transversely across said first surface into saidfirst region and terminating short of said junction, said second regionhaving sensitivity enhancing point defects in the area of said junctionclosest to said groove,

(0) first and second electrodes electrically connected respectively tosaid first and second regions for connection of the device into anexternal circuit,

(d) and mounting means connected to one end of the body for supportingthe body in a cantilevered position for bending movement about the apexof the groove.

2. A device as set forth in claim 1 wherein said groove ischannel-shaped in cross section.

3. A device as set forth in claim 1 wherein said groove is V-shaped incross section.

4. A sensitized strain transducer comprising (a) an axially extendingsemiconductor body having a first region of selected conductivity typeand a second region of opposite conductivity type, said first and secondregions having respective first and second oppositely disposed planarsurfaces, and a P-N junction therein between said regions extending in aplane parallel with one of said surfaces,

(b) a groove extending transversely across said first surface of saidsemiconductor body into said first region and terminating short of saidjunction and oriented perpendicular to the axis of said body,

(c) said second region having sensitivity enhancing point defectstherein in the area of said junction closest to said groove, and

(d) electrode support means engaged with the semiconductor bodymaintaining said body in a cantilevered position and providingelectrical contact means for connecting said transducer to an externalcircuit.

References Cited by the Examiner UNITED STATES PATENTS ANTHONY BARTIS,

Hoesterey.

Muller.

Shockley.

Doucette et al. 338-2 Steigerwald.

Wright 3382 Fell 3382 X McLellan 338--4 Rindner 338-2 Primary Examiner.

RICHARD M. WOOD, Examiner.

W. D. BROOKS, Assistant Examiner.

1. A DEVICE COMPRISING (A) A BODY OF SEMICONDUCTOR MATERIAL HAVING AFIRST REGION OF SELECTED CONDUCTIVITY TYPE AND A SECOND REGION OFOPPOSITE CONDUCTIVITY TYPE, SAID FIRST AND SECOND REGIONS HAVINGRESPECTIVE FIRST AND SECOND OPPOSITELY DISPOSED PLANAR SURFACES, AND AP-N JUNCTION THEREIN BETWEEN SAID REGIONS AND EXTENDING IN A PLANEPARALLEL WITH ONE OF SAID SURFACES, (B) A GROOVE EXTENDING TRANSVERSELYACROSS SAID FIRST SURFACE INTO SAID FIRST REGION AND TERMINATING SHORTOF SAID JUNCTION, SAID SECOND REGION HAVING SENSITIVITY ENHANCING POINTDEFECTS IN THE ARREA OF SAID JUNCTION CLOSEST TO SAID GROOVE, (C) FIRSTAND SECOND ELECTRODES ELECTRICALLY CONNECTED RESPECTIVELY TO SAID FIRSTAND SECOND REGIONS FOR CONNECTION OF THE DEVICE INTO AN EXTERNALCIRCUIT, (D) AND MOUNTING MEANS CONNECTED TO ONE END OF THE BODY FORSUPPORTING THE BODY IN A CANTILEVERED POSITION FOR BENDING MOVEMENTABOUT THE APEX OF THE GROOVE.